1. Field of the Invention
The present invention relates to a display device in which light emitting elements, such as organic electroluminescence (EL) elements whose luminescent brightness is controlled by electric current, are provided per pixel, and particularly to an active matrix type display device in which the quantity of electric current to be supplied to each light emitting element is controlled by an active element such as a field effect transistor and which can reproduce the display brightness irrespective of variations in characteristics of the active element.
2. Background Art
An organic EL display device is a self-emission type display device in which an organic EL element that serves as a light emitting element is provided per pixel, and it has advantages such as high visibility of images, no need for a back light, and fast response speed, as compared with a liquid crystal display device. Further, since the luminescence brightness of the organic EL element is controlled by the value of the driving electric current, it is necessary that an electric current having a magnitude corresponding to the brightness information be applied to the organic EL elements for respective pixels.
Meanwhile, the driving system of the organic EL display device includes a simple matrix type and an active matrix type. The former type is simple in structure, but makes it difficult to achieve a large screen and high image resolution since it emits light only for a scanning period, and the latter type, i.e., the active matrix type, is more advantageous for achieving a large screen and high-resolution of an image. In the active matrix type, the current to be applied light emitting elements provided per pixel is controlled by an active element such as a transistor in the pixel. In the case of the organic EL display device, such an active element is realized by a thin film transistor (TFT: Thin Film Transistor).
FIG. 1 is a schematic structural view of a conventional active matrix type organic EL display device. In an organic EL panel 10 are provided a plurality of horizontal scanning lines Scan 1 to N, a plurality of vertical data lines Data 1 to M, and matrix-like pixels PX arranged at intersecting points therebetween. A scanning line driving circuit 14 scans successively the scanning lines Scan 1 to N within a frame period, and a data line driving circuit 12 supplies electric current corresponding to the brightness information to the pixels through the data lines Data during each scanning period.
FIG. 2 is a view showing an example of a pixel circuit of a conventional organic EL element. The pixel circuit is described in Japanese Patent Application Laid-Open (JP-A) No. 8-234683 (hereinafter, referred to as Patent Document No. 1). Further, an analogous pixel circuit is described in “Passive and active matrix addressed polymer light emitting diode displays”, SPIE2001, PLED, final (hereinafter, referred to as Non-Patent Document No. 1).
This pixel circuit comprises an N-channel transistor TFT1, which is on-off controlled by a scanning line Scan, a P-channel transistor TFT2 for driving an organic EL element OLED, and a storage capacitor C provided between the gate of the transistor TFT2 and a power source Vdd.
The operation of the pixel circuit is carried out as described below. When the transistor TFT1 is turned on with the scanning line Scan in selected state so that a data potential Vdata corresponding to the brightness information is applied to the data line Data, the capacitor C is charged or discharged through the transistor TFT1, and a potential corresponding to the data potential Vdata is accumulated at the gate node Nd of the transistor TFT2. Thereafter, when the transistor TFT 1 is turned off with the scanning line Scan in non-selected state, the transistor TFT2 flows a drain-source current Ids2 corresponding to the potential at the gate node Nd so that the light emitting element OLED emits light with brightness corresponding to the drain-source current Ids2. The drain-source current Ids2 depends on the gate-source voltage Vgs of the transistor TFT2 (=the potential at the gate node Nd−voltage at the OLED). Meanwhile, the transistor TFT2 is operated in a saturated region, so that the drain-source current Ids2 is controlled only by the gate-source voltage Vgs even if unevenness is caused to occur in the Vds of the transistor TFT2 due to the unevenness in characteristic of the light emitting element OLED.
Through use of the pixel circuit described above, as shown in FIG. 1, brightness information can be written by charging or discharging the capacitor C for each pixel in the scanning period and the light emitting element for each pixel is operated according to the written information, during the subsequent read-out period. Consequently, the driving current of the light emitting element can be decreased by prolonging the light emission period of the light emitting element, and thus it is possible to achieve a display device having a large screen and high resolution.
In the pixel circuit shown in FIG. 2, there is a problem of variation of brightness among pixels attributed to the variation in the characteristics of TFT formed on display panel. The TFT is formed on a substrate such as glass, and due to the production variations, the threshold voltage of TFT and carrier mobility vary, and correspondingly, the drain-source current Ids2 of the transistor TFT2 also vary. Due to the unevenness of the drain-source current Ids2, which is the driving current, the luminescence brightness of the light emitting element OLED becomes uneven.
In order to make luminescence brightness independent of the characteristic unevenness of TFT such as described above, a pixel circuit shown in FIG. 3 has been proposed, which, for example, is disclosed in JP-A No. 2001-147659 (hereinafter, referred to as Patent Document No. 2) and “Pixel-Driving Methods for Large-Sized Poly-Si AM-OLED Displays” Asia Display/IDW 2001, OEL 1-1, p 1395 (hereinafter, referred to as Non-Patent Document No. 2). The pixel circuit comprises a transistor TFT3 controlled by a scanning line ScanA, a transistor TFT4 controlled by a scanning line ScanB, transistors TFT1 and TFT2 with their gates connected in common, a capacitor provided between a common gate Nd and a constant voltage terminal Vdd and a light emitting element OLED is current operated by the transistor TFT2.
According to the description in the above-mentioned Patent Document No. 2, the operation of the pixel circuit shown in FIG. 3 is like this. When brightness information is written, the transistor TFT3 is turned on with the scanning line ScanA in the selected state (H level), and the transistor TFT4 is also turned on with the scanning line ScanB in the selected state (L level), so that the electric current Idata corresponding to the brightness is caused to flow through the data line and thus the current Iw corresponding to the brightness is passed to the transistor TFT1. The transistor TFT1 is in a saturated state due to the drain-gate thereof being short-circuited by the transistor TFT4, and a current mirror circuit is formed. The capacitor C is charged by the drain source current Iw and a potential corresponding to the brightness information is written in the node Nd. On the other hand, at the time of reading out, both scanning lines ScanA and ScanB are in non-selected state and both of the transistors TFT3 and TFT4 are turned off. At this time, the transistor TFT2 supplies drain-source electric current Ids2 corresponding to the gate potential to the light emitting element OLED and causes it to emit light. The drain-source electric current Ids2 has a relation with the electric current Iw corresponding to the brightness information such that the current value corresponds to a ratio of the gate width and the gate length of the transistors TFT1 and TFT2. Thus, the light emitting element OLED can be operated with the driving current Ids2 corresponding to the electric current Iw at the time of writing, causing the light emitting element OLED to emit light with luminescence brightness corresponding to the brightness information.